DC-DC converter selection

I’m trying to find an efficient means of regulating the output from a battery for a few of my projects but I’m not sure which configuration would result in the longest battery life for the lowest cost. I would like to output a steady 2.5v to 3.3v from two AA batteries, one project draws < 10mA and the other is probably in the same ballpark although I haven’t finished it yet. I believe for maximum battery life I would have to go with a switching regulator. As I need a steady 2.5v-3.3v source I would need to have the voltage reduced with a fresh battery and boosted with older batteries. I think I have three options but I don’t know which one is the most efficient. I could use a SEPIC converter and connect the batteries in series, a step-up converter with the batteries connected in parallel, or a step-up/down converter with a built in LDO and the batteries connected in series. I was reading the Maxim application notes and they mentioned that the step-up/LDO combo was actually more efficient than the SEPIC for loads over about 7mA (Step-up Converter with LDO Beats SEPIC Efficiency - Maxim). I was considering using the MAX710/1 which uses the LDO when Vin>Vout and the boost converter when Vin<Vout. I didn’t know if it would kill my battery until Vin < Vout however, and the chips cost around $8 each which is a little more than I would like to spend. The SEPIC just seems a little complicated to me but if it was the best option I would consider it. What I really would like to know is weather a two parallel batteries and a boost converter would be more efficient than two batteries in series with the step-up/LDO combo or if I should attempt a SEPIC converter.